The invention relates to lithography instruments used for patterning and processing substrates such as semiconductor chips and wafers. More specifically, the invention is concerned with an apparatus and method for using interferometers to determine the position of substrate stages during the simultaneous processing of the substrates affixed to these stages.
Lithography processes require positioning a reticle between an electron beam and the substrate chip or wafer. System throughput is dependent upon the speeds of many separate processes that are performed in series. Throughput is therefore dependent on the duration of each process.
In a typical modern lithography process an individual wafer undergoes a number of sub-processes. These can include: loading, field image alignment, global alignment, and exposure. The production of an acceptable final product requires the complex interaction of the systems necessary to implement each sub-process. For example, in the sub-process for exposing patterns on wafers and other substrates, the reticle is moved at high speeds between discrete and precise positions to facilitate focusing the image on the substrate. This motion can generate dynamic reaction forces where the reticle is supported, leading to distortion of the reticle and, hence, distortion of the image focused on the substrate. Both reticle and wafer must be held without slippage and in a way that does not cause distortion of the reticle pattern. The system is further complicated by the fact that lithography processes typically occur in a clean room/vacuum environment; this is also an indication of the sensitivity of the processes.
A typical exposure apparatus 10 employing a single wafer stage is shown in FIG. 1 and FIG. 2. Exposure apparatus 10 transfers a pattern of an integrated circuit from reticle 12 onto semiconductor wafer 14. Apparatus frame 16 preferably is rigid and supports the components of exposure apparatus 10. These components include: reticle stage 18, wafer stage 20, lens assembly 22, and illumination system 24. Alternatively, separate, individual structures (not shown) can be used to support wafer stage 20, reticle stage 18, illumination system 24, and lens assembly 22.
Illumination system 24 includes an illumination source 26 and an illumination optical assembly 28. Illumination source 26 emits an exposing beam of energy such as light or electron energy. Optical assembly 28 guides the beam from illumination source 26 to lens assembly 22. The beam illuminates selectively different portions of reticle 12 and exposes wafer 14. In FIG. 1, illumination source 26 is illustrated as being supported above reticle stage 18. Typically, however, illumination source 26 is secured to one of the sides of apparatus frame 16 and the energy beam from illumination source 26 is directed to above reticle stage 18 with illumination optical assembly 28. Where illumination source 26 is an electron beam, the optical path for the electron beam should be in a vacuum.
Lens assembly 22 projects and/or focuses the light passing through reticle 12 to wafer 14. Depending upon the design of apparatus 10, lens assembly 22 can magnify or reduce the image illuminated on reticle 12.
Reticle stage 18 holds and precisely positions reticle 12 relative to lens assembly 22 and wafer 14. Similarly, wafer stage 20 holds and positions wafer 14 with respect to the projected image of the illuminated portions of reticle 12. In the embodiment illustrated in FIG. 1 and FIG. 2, wafer stage 20 and reticle stage 18 are positioned by shaft-type linear motors 30. Depending upon the design, apparatus 10 may include additional servo drive units, linear motors and planar motors to move wafer stage 20 and reticle stage 18, but other drive and control mechanisms may be employed.
The basic device as described may be used in different types of lithography processes. For example, exposure apparatus 10 can be used in a scanning type lithography system, which exposes the pattern from reticle 12 onto wafer 14 with reticle 12 and wafer 14 moving synchronously. In a scanning type lithography process, reticle 12 is moved perpendicular to an optical axis of lens assembly 22 by reticle stage 18, and wafer 14 is moved perpendicular to an optical axis of lens assembly 22 by wafer stage 20. Scanning of reticle 12 and wafer 14 occurs while reticle 12 and wafer 14 are moving synchronously.
Alternatively, exposure apparatus 10 may be employed in a step-and-repeat type lithography system that exposes reticle 12 while reticle 12 and wafer 14 are stationary. In the step-and-repeat process, wafer 14 is in a constant position relative to reticle 12 and lens assembly 22 during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer 14 is consecutively moved by wafer stage 20 perpendicular to the optical axis of lens assembly 22 so that the next field of semiconductor wafer 14 is brought into position relative to lens assembly 22 and reticle 12 for exposure. Following this process, the images on reticle 12 are sequentially exposed onto the fields of wafer 14.
This complexity and sensitivity of the exposure apparatus and the processes involved result in a significant time expenditure for each sub-process. When a single wafer is undergoing one of these sub-processes, the mechanisms for the others are normally idle. Consumer demand for the end product has created a need for increased throughput and, thus, the development of methods to decrease the idle time. One current method uses two stages that run simultaneously, but with each stage at different steps in the process. This method relies upon a combination of encoders and interferometers to determine the position of each stage at any given point throughout processing.
Encoders, however, are less than ideal devices for this use for a number of reasons. The encoder must be placed in an area that does not interfere with the requirements of other sub-processes, such as substrate exposure. This leads to apparatus design problems in harmonizing the requirements of the encoder, interferometers, and the process. Also, encoders are less precise than interferometers. Precision in planar placement of the stage is necessary, since errors in reticle or wafer position result in similar errors in the final product and, therefore, reduced functionality of that final product.
The present invention provides a dual stage assembly and method where stage position may be determined using interferometers. The stage assembly includes a plurality of interferometers mounted on a base for determining stage positions. The two stages move between multiple positions on the base and have mirrors affixed to them that cooperate with the other interferometer components to provide position data. At times, the two stages are positioned so that the first stage eclipses the second stage with respect to said at least one of the interferometers. Whenever such an eclipse occurs, the mirror on the second (eclipsed) stage is configured to cooperate with the non-eclipsed interferometers so that the position of said second stage is continuously determinable. This is achieved by appropriately dispersing the interferometers about one side of the base and by causing the mirror on the second stage to extend from behind the eclipsing shadow of the first stage. In a preferred embodiment, the second stage is the same size as the first and merely supports the larger mirror. In another preferred embodiment, the second stage is approximately the same size as the mirror in the relevant dimensions. In both the previously mentioned preferred embodiments the stages are the same size in the direction parallel to the axes of the interferometers. But the invention could also be practiced in two dimensions resulting in the need for interferometers on only two orthogonal sides of the base.
A method incorporating the invention comprises: sizing the stages based on wafer and exposure apparatus parameters; dispersing interferometers about the sides of the base at appropriate positions based on the stage sizes; configuring the mirror on the second stage to continue to cooperate with enough other interferometer elements to provide position data even if the first stage is positioned between the second stage and some of the interferometers; moving the stages as desired during the course of using the exposure apparatus; and determining the positions of both stages at all times during the process. A preferred embodiment of the invention practices the method with respect to one dimension of the apparatus; resulting in interferometers on three sides of the base. Another embodiment of the invention practices the method with respect to both dimensions of the plane of movement; resulting in interferometers on two orthogonal sides of the base.